The demand for advanced digital devices having high-speed, high-frequency and high-functional characteristics with a reduced size has recently been increasing, in addition to the increased flexibility required for flexible electronic devices. In order to meet such demand, there is a tendency to incorporate into a device hybrid active/passive elements radiating a dielectric noise. However, such hybridization may increase linear coupling due to capacitive coupling as well as inductive coupling and cause interference due to the action of the radiation noise, which tends to cause frequent malfunction of the device. In some cases, such noise may negatively affect other external devices.
To suppress the radiation noise, particularly in the range of from 100 MHz to 20 GHz, and the electromagnetic interference for such advanced devices, there have been developed a method of using a low pass filter and shielding. However, the functional part used in such method requires a large mounting space, and accordingly, such method is not suitable for a small digital device or a product having a short life. Furthermore, inductance parts are unsuitable for use in the Radio Frequency (Hereinafter, refer to “RF”) range.
In response to the trend of reducing the size of a digital device, the demand for developing an ultra thin noise suppressing film usable in the RF range has been increasing. A conventional electromagnetic shielding material containing Cu, Ag, or others is applied mainly on the cover of a digital device or is mounted on the housing thereof so as to shield undesirable electromagnetic waves coming from outside. However, such a conventional material cannot deal effectively with the radiation noise as well as conductive noise occurring between the circuits within the device. Also, in case an electromagnetic absorbing sheet of a ferrite or sendust alloy having good shielding properties is used, the thickness thereof cannot be reduced to a satisfactory level because the magnetic permeability drastically decreases with the increase in the frequency. In the frequency range of tens to hundreds of MHz, parts such as a coil or filter may be used, but not in the above mentioned RF range. Even if passive parts useful in the RF range are provided, immense costs are required to change the design of the substrate and others.
The above-mentioned problems may be solved through the development of a noise-suppressing film containing a magnetic powder capable of suppressing unwanted electromagnetic noises. Such a film may be used at a frequency range higher and wider than hundreds of MHz has been possible, if it can be fabricated in the form of an ultra thin film. Accordingly, there is a need to develop such a thin noise suppressing film.
With regard to conventional noise suppressing films, Japanese Patent Publication No. 2005-019846 discloses a method for manufacturing a noise suppressing film using a spherical or crushed soft magnetic metal oxide powder of a magnetic ferrite or sendust alloy having an average diameter of 0.1˜10 μm. However, the soft magnetic metal oxide powder used therein produces unsatisfactorily low responses at a high frequency range. Also, the literature (‘Crystal structure and microwave permeability of very thin Fe—Si—Al flakes produced by microforging’, Shigeyoshi Yoshida, Journal of applied physics, 93[10], 2003, pp. 6659) describes a method for manufacturing a noise suppressing film using Fe—Si—Al flakes as an electromagnetic absorbing material. Although an enhanced frequency response of the soft magnetic alloy is attempted by performing high-energy milling, the high frequency response in the desired RF range has remained unsatisfactory.
Thus, the conventional materials cannot effectively suppress unneeded electromagnetic waves generated by advanced devices having a reduced size. In case of an ultra thin device, it also becomes very difficult to effectively cope with the heat emitted from various parts of the device such as printed circuit board (PCB) and flexible PCB (FPCB). Therefore, there is a need to develop a thin functional electromagnetic composite film which is able to overcome the limitations of the conventional materials and withstand a high-temperature condition generated by the heat emitted by various parts in a down-sized device.